The present invention generally relates to connection hold control systems, and more particularly to a connection hold control system which holds a channel connection during a data communication between terminals which are respectively coupled to a communication network via a terminal adapter.
A terminal may be provided with an interface which can be connected directly to a network such as a public communication network. But if the terminal cannot be connected directly to the communication network, the terminal is coupled to the communication network via a terminal adapter and the communication is made by exchanging protocols.
For example, a data communication may be made by coupling a terminal which is provided with a high performance parallel interface (HIPPI) to a communication network such as a broadband integrated services digital network (B-ISDN) which transmits data in the asynchronous transfer mode (ATM). The HIPPI is used when transmitting data in parallel in one direction at a high speed. The B-ISDN which employs the ATM will hereinafter be referred to as the ATM network. In this case, a terminal adapter (TA) is interposed between the terminal and the ATM network. The HIPPI protocol of the transmitting terminal must be converted into the ATM protocol in the transmitting side terminal adapter so as to carry out an exchange/transmission within the ATM network, and the receiving side terminal adapter must again convert the ATM protocol into the HIPPI protocol before transferring it to the receiving terminal. Although the HIPPI is designed for the terminal which transmits in one direction at a high speed, the control information only is transmitted from the receiving side to the transmitting side. Hence, a conversion similar to the above is also carried out with respect to the control information in the reverse direction.
FIG. 1 is a diagram for explaining the network structure described above. In FIG. 1, three terminals 62-1 through 62-3 (TE#1 through TE#3) are coupled to an ATM network 60 via respective terminal adapters 61-1 through 61-3. The three terminals 62-1 through 62-3 are respectively provided with the HIPPI, and the HIPPIs are interposed between the terminals 62-1 through 62-3 and the corresponding terminal adapters 61-1 through 61-3.
When making a communication among the terminals 62-1 through 62-3, the terminal adapter converts the HIPPI protocol into the ATM protocol and makes the communication via the ATM network 60.
In order to transmit data in the network structure described above, a connection is set by setting from the transmitting side terminal adapter a communication path (hereinafter referred to as a channel) from the transmitting terminal to the receiving terminal via the ATM network 60. For example, if one terminal 62-1 shown in FIG. 1 it to alternately transmit data to the two terminals 62-2 and 62-3, the terminal adapter 61-1 must release the previous channel every time a request to switch the destination is generated from the transmitting terminal 62-1 and a channel must be set with respect to the next destination.
However, a certain time is required to set the connection. The time required to set the connection becomes large particularly in the case of a data communication in which the destination is switched frequently, and the data communication efficiency becomes poor in this case. For this reason, there is a demand to realize a method of setting the connection which can improve the data communication efficiency.
FIG. 2 shows the structure of a conventional terminal adapter which is used between the ATM network and the terminal which is provided with the HIPPI. FIG. 3 is a sequence diagram for explaining the operation of the prior art.
First, a description will be given of a method of setting a connection in the HIPPI.
The transmitting terminal (not shown) makes a connection request with respect to the transmitting side terminal adapter in order to transmit data, and this connection request is made by turning ON the REQUEST shown in FIG. 2. At the same time, data (hereinafter referred to as connection information) including destination information and the like are transmitted on a data line DATA to the terminal adapter in 32 parallel bits, for example. A data part in which the connection information is transmitted is called an I field, and the actual data is transmitted following this I field.
At the transmitting side terminal adapter shown in FIG. 2, a HIPPI connection management part 71 receives the above information via a transmission interface part 72 and carries out a control so as to turn ON the REQUEST and send the REQUEST to an ATM cell generating part 74 together with the connection information which is input to a transmission buffer 73. According to the ATM, the control information and data are transferred in units of cells respectively made up of 53 bytes, as is well known. The REQUEST which is ON and the data (connection information) of the I field are formed into one cell in the ATM cell generating part 74, and is transmitted to the ATM network (not shown) via a physical layer transmission processing part 78. This cell is thus transmitted to the receiving side terminal adapter (not shown) and the receiving terminal (not shown) via the ATM network. The transmission buffer 73 and a reception buffer 77 are normally made up of a first-in-first-out (FIFO) which stores and outputs the incoming data.
When the receiving terminal receives the cell from the transmitting terminal via the ATM network, the receiving terminal turns ON the CONNECT and returns the CONNECT to the transmitting side. The CONNECT is received as a cell by an ATM cell reception processing part 75 via a physical layer reception processing part 79 shown in FIG. 2, and this cell is converted into the HIPPI format before being sent to a HIPPI connection management part 71. The HIPPI connection management part 71 turns ON the CONNECT which is supplied to the transmitting terminal via the transmission interface part 72. The CONNECT which is ON indicates a connection set completion signal.
When using the terminal adapter shown in FIG. 2 on the receiving side, the connection information, data and the like which are received are sent from the ATM cell reception processing part 75 to the receiving terminal via the reception buffer 77 and a reception interface part 76.
When the data transmission ends, the transmitting terminal turns OFF the REQUEST, and this REQUEST which is OFF is regarded as a connection release request. The OFF REQUEST is transmitted to the receiving terminal similarly as described above, and the receiving terminal turns OFF the CONNECT when the OFF REQUEST is received. The OFF CONNECT is transmitted to the transmitting terminal similarly as described above, and the OFF CONNECT is regarded as a connection release acknowledgement signal.
FIG. 3 shows the sequence for the network structure shown in FIG. 2 in a case where the data is transmitted from the terminal TE#1 to the terminals TE#2 and TE#3. In FIG. 3, REQ-1/REQ-0 indicates the REQUEST which is ON/OFF, and CON-1/CON-0 indicates the CONNECT which is ON/OFF. In addition, REQ-1+I indicates that the connection information is transmitted in the I field together with the REQ-1. Parts of the sequence are denoted by "a" through "q" with the exception of letters "l" and "o" which are not used.
First, when it is assumed that the data transmission is made from the terminal TE#1 to the terminal TE#2, the terminal TE#1 makes a connection request by REQ-1 as shown by "a" and notifies the terminal adapter TA#1 that the destination is the terminal TE#2 by the I field. The REQ-i+I is formed into a cell in the terminal adapter TA#1, and is sent to the terminal adapter TA#2 via the ATM network. The terminal adapter TA#2 sends the cell which is received via the ATM network to the terminal TE#2. The terminal TE#2 returns CON-1 to the terminal TE#1 as shown by "b"to confirm the receipt of the REQ-1 via a reverse route. As a result, a channel is set between the terminals TE#1 and TE#2, and it becomes possible to transmit data from the terminal TE#1 to the terminal TE#2 as shown by "c".
When the data transmission ends, the terminal TE#1 makes a connection release request by REQ-0 and CON-0 is returned from the receiving side similarly as described above as shown by "d" and "e". Hence, the channel between the terminals TE#1 and TE#2 becomes disconnected.
Next, when the data transmission is made from the terminal TE#1 to the terminal TE#3, a sequence similar to that described above is carried out as shown by "f" through "j". Furthermore, if the data is to be transmitted to the terminal TE#2 again after the data transmission from the terminal TE#1 to the terminal TE#3, a sequence is carried out as shown by "k" through "q".
Therefore, a time .tau. for setting and releasing the channel is required with respect to a time .alpha. in which the data is transmitted every time a data transmission is made. For this reason, when alternately transmitting data to a plurality of terminals as described above, the time required to set and release the channel becomes large relative to the data transmission time, and there is a problem in that the data transmission efficiency becomes poor.